1. Field of the Invention
The present invention relates to the control of switched reluctance machines, particularly those machines which are operated without a sensor to measure rotor position.
2. Description of Related Art
In general, a reluctance machine is an electrical machine in which torque is produced by the tendency of its movable part to move into a position where the reluctance of a magnetic circuit is minimized, i.e. where the inductance of the exciting winding is maximized. In one type of reluctance machine, circuitry is provided for detecting the angular position of the rotor and energizing the phase windings as a function of the rotor position. This is generally known as a switched reluctance machine and it may be operated as a motor or a generator. The characteristics of such switched reluctance machines are well known and are described in, for example, "The characteristics, design and application of switched reluctance motors and drives" by Stephenson and Blake, PCIM'93, Nurnberg, 21-24 June 1993, incorporated herein by reference. That paper describes in some detail the features of the switched reluctance machine which together produce the characteristic cyclically varying inductance of the phase windings.
FIG. 1 shows the principal components of a typical switched reluctance drive system. The input DC power supply 11 can be either a battery or a rectified and filtered AC supply and can be fixed or variable in magnitude. In some known drives, the power supply 11 includes a resonant circuit which produces a DC voltage which rapidly varies between zero and a predetermined value to allow zero voltage switching of power switches. The DC voltage provided by the power supply 11 is switched across the phase windings 16 of the motor 12 by a power converter 13 under the control of the electronic control unit 14. The switching must be correctly synchronized to the angle of rotation of the rotor for proper operation of the drive. A rotor position detector 15 is typically employed to supply signals indicating the angular position of the rotor. The output of the rotor position detector 15 may also be used to generate a speed feedback signal.
The rotor position detector 15 may take many forms, for example it may take the form of hardware, as shown schematically in FIG. 1. In some systems, the rotor position detector 15 can comprise a rotor position transducer that provides output signals that change state each time the rotor rotates to a position where a different switching arrangement of the devices in the power converter 13 is required. In other systems, the position detector can be a software algorithm which calculates or estimates the position from other monitored parameters of the drive system. These systems are often called "sensorless position detector systems" since they do not use a physical transducer associated with the rotor which measures the position. As is known in the art, many different approaches have been proposed in the quest for a reliable sensorless system. Some of these approaches are discussed below.
The energization of the phase windings in a switched reluctance machine depends on detection of the angular position of the rotor relative to the stator. This may be explained by reference to FIGS. 2 and 3, which illustrate the switching of a reluctance machine operating as a motor. FIG. 2 generally shows a rotor 24 with a rotor pole 20 approaching a stator pole 21 of a stator 25 according to arrow 22. As illustrated in FIGS. 2 and 3, a portion 23 of a complete phase winding 16 is wound around the stator pole 21. When the portion 23 of the phase winding 16 around stator pole 21 is energized, a force will be exerted on the rotor, tending to pull rotor pole 20 into alignment with stator pole 21. FIG. 3 generally shows typical switching circuitry in the power converter 13 that controls the energization of the phase winding 16, including the portion 23 around stator pole 21. When switches 31 and 32 are closed, the phase winding is coupled to the source of DC power and is energized. Many other configurations of lamination geometry, winding topology and switching circuitry are known in the art: some of these are discussed in the Stephenson & Blake paper cited above. When the phase winding of a switched reluctance machine is energized in the manner described above, the magnetic field set up by the flux in the magnetic circuit gives rise to the circumferential forces which, as described, act to pull the rotor poles into line with the stator poles.
In general, the phase winding is energized to effect the rotation of the rotor as follows. At a first angular position of the rotor (called the "turn-on angle", .theta..sub.ON), the controller 14 provides switching signals to turn on both switching devices 31 and 32. When the switching devices 31 and 32 are on, the phase winding is coupled to the DC bus, causing an increasing magnetic flux to be established in the machine. The magnetic flux produces a magnetic field in the air gap which acts on the rotor poles to produce the motoring torque. The magnetic flux in the machine is supported by the magneto-motive force (mmf) which is provided by a current flowing from the DC supply through the switches 31 and 32 and the phase winding 16. Current feedback is typically employed and the magnitude of the phase current is controlled by chopping the current by rapidly switching one or both of switching devices 31 and/or 32 on and off. FIG. 4(a) shows a typical current waveform in the chopping mode of operation, where the current is chopped between two fixed levels. In motoring operation, the turn-on angle .theta..sub.ON is often chosen to be the rotor position where the center-line of an inter-polar space on the rotor is aligned with the center-line of a stator pole, but may be some other angle.
In many systems, the phase winding remains connected to the DC bus (or connected intermittently if chopping is employed) until the rotor rotates such that it reaches what is referred to as the "freewheeling angle", .theta..sub.FW. When the rotor reaches an angular position corresponding to the freewheeling angle (e.g., the position shown in FIG. 2) one of the switches, for example 31, is turned off. Consequently, the current flowing through the phase winding will continue to flow, but will now flow through only one of the switches (in this example 32) and through only one of the diodes 33/34 (in this example 34). During the freewheeling period, the voltage drop across the phase winding is small, and the flux remains substantially constant. The circuit remains in this freewheeling condition until the rotor rotates to an angular position known as the "turn-off angle", .theta..sub.OFF, (e.g. when the center-line of the rotor pole is aligned with that of the stator pole). When the rotor reaches the turn-off angle, both switches 31 and 32 are turned off and the current in phase winding 23 begins to flow through diodes 33 and 34. The diodes 33 and 34 then apply the DC voltage from the DC bus in the opposite sense, causing the magnetic flux in the machine (and therefore the phase current) to decrease. It is known in the art to use other switching angles and other current control regimes.
As the speed of the machine rises, there is less time for the current to rise to the chopping level, and the drive is normally run in a "single-pulse" mode of operation. In this mode, the turn-on, freewheel and turn-off angles are chosen as a function of, for example, speed and load torque. Some systems do not use an angular period of freewheeling, i.e. switches 31 and 32 are switched on and off simultaneously. FIG. 4(b) shows a typical such single-pulse current waveform where the freewheel angle is zero. It is well known that the values of turn-on, freewheel and turn-off angles can be predetermined and stored in some suitable format for retrieval by the control system as required, or can be calculated or deduced in real time.
It will be realized that sensorless position detection systems have to be capable of providing rotor position signals in both chopping and single-pulse operating modes if the full capabilities of the switched reluctance machine are to be realized. Though many sensorless systems have been proposed, they tend to be limited to either one mode of operation or to impose severe restrictions on the operation of the System. One proposed method using diagnostic pulses in idle phases is described in "A new sensorless position detector for SR drives" by Mvungi et al., Proc. PEVD Conf., IEE Pub'n No. 324, London, Jul. 17-19, 1990, pp. 249-252, which is incorporated herein by reference. Typically this approach is successful in the chopping mode, where the rise and fall times of the current are relatively short compared with the overall excitation cycle. The paper concedes that a different approach is required for high-speed (i.e. single-pulse) operation.
An approach for high speed operation is exemplified by EP-A-0573198 (Ray), incorporated herein by reference, which discloses a method of flux and current measurement leading to predictions of rotor position. Many other sensorless position detection systems are reviewed and categorized in "Sensorless methods for determining the rotor position of switched reluctance motors", Ray et al., Proc. EPE '93 Conference, Brighton, UK, Sep., 13-16 1993, Vol. 6, pp. 7-13, incorporated herein by reference, and it was concluded in that article that none of these methods was entirely satisfactory for operation over both operating ranges.